In digital, magnetic and optical storage systems for audio/video/data, PLL (Phase Locked Loop) systems are used in playback mode for the purpose of resynchronizing the clock pulse from the data. For the purpose of reconverting the digital data from the analog-sampled signals, provision is made, in these systems, for decision devices which, in the case of the novel systems such as DVC (recording on tape) or DVD (optical playback), are designed as Viterbi detectors in a self-adapting form which is optimally matched to the type of storage medium. The Viterbi detector operates in general in a digital fashion, that is to say the signal which is present in analog form is digitized with the aid of an A/D (analog/digital) converter and fed to the detector.
It is disadvantageous in this case that the sampling instant is influenced by the response of the PLL and the equalization. It is, furthermore, dependent on distortions which have ocurred during the recording, as well as on the thermal response of the electronics. In reality, a variance of approximately 10% is to be expected. A non-optimum sampling instant "at the centre of the eye pattern", however, leads to more faulty decisions in the Viterbi detector, which it corrects automatically to a certain degree. The error correction will, for the most part, correct the remainder still left over, it being possible for the correction capacity to be quickly exceeded in the case of additional amplitude dips owing to scratches on the tape or blemishes in the tape. The result is then loss of data.